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Stock award plan - Share Base Compensation (Details) - Stock option
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Number of options  
Outstanding at the beginning of the period (in shares) | shares 6,448,642
Granted (in shares) | shares 1,916,939
Exercised (in shares) | shares (77,312)
Forfeited/Cancelled (in shares) | shares (111,492)
Outstanding at the end of the period (in shares) | shares 8,176,777
Vested or Expected to vest at the end of the period (in shares) | shares 3,711,261
Exercisable at the end of the period (in shares) | shares 4,166,448
Weighted- average exercise price  
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 29.00
Granted (in dollars per share) | $ / shares 18.71
Exercised (in dollars per share) | $ / shares 14.70
Forfeited/Cancelled (in dollars per share) | $ / shares 34.42
Outstanding at the end of the period (in dollars per share) | $ / shares 26.65
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares 21.15
Exercisable at the end of the period (in dollars per share) | $ / shares $ 32.12
Weighted- average remaining contractual term  
Outstanding at the end of the period 7 years 8 months 1 day
Vested or Expected to vest at the end of the period 8 years 11 months 23 days
Exercisable at the end of the period 6 years 4 months 24 days
Aggregate intrinsic value  
Outstanding at the end of the period (in dollars) | $ $ 56,205
Vested or Expected to vest at the end of the period (in dollars) | $ 32,239
Exercisable at the end of the period (in dollars) | $ $ 21,177
Minimum  
Valuation assumptions  
Risk-free interest rate (as a percent) 2.25%
Expected volatility (as a percent) 64.00%
Expected term 5 years 16 days
Maximum  
Valuation assumptions  
Risk-free interest rate (as a percent) 2.73%
Expected volatility (as a percent) 90.00%
Expected term 10 years